1. Field of the Invention
The present invention relates to a malfunction determining apparatus of an exhaust gas recirculation system and, more particularly, to a malfunction determining apparatus of an exhaust gas recirculation system which detects a malfunction of an EGR valve of the exhaust gas recycling system.
2. Description of the Related Art
An exhaust gas recirculation system (hereinafter referred to as an EGR) is known as means for purifying exhaust gas of an internal combustion engine (hereinafter referred to as an engine). The EGR reduces NOx by recirculating a part of exhaust gas to an intake line.
In an engine having the EGR, since the amount of smoke is increased as the recirculation rate (a ratio of an exhaust gas to a fresh gas) is increased, the amount of recirculation of exhaust gas cannot be increased beyond a certain limit. Additionally, since the amount of smoke tends to be increased as the load of the engine is increased, a target exhaust gas recirculation rate is varied in response to operating conditions of the engine by providing an exhaust gas recirculating valve (EGR valve) in an exhaust gas recirculation passage. An open/close operation of the EGR valve is controlled in response to the target exhaust gas recirculation rate so that the maximum recirculation of exhaust gas can be achieved within a range in which an excessive amount of smoke is not generated.
In a case where the above-mentioned EGR valve is fixed in an open state due to malfunctioning, exhaust gas is continuously recirculated to an intake line. This may increase the amount of smoke and cause an engine stall. Accordingly, a malfunction determining apparatus has been suggested to detect a malfunction of the EGR.
A malfunction determining apparatus of the above-mentioned type is disclosed in Japanese Laid-Open Patent Application No. 4-103865. The malfunction determining apparatus of the EGR disclosed in the above-mentioned publication detects an actual degree of opening of the EGR valve (actual valve opening degree), and sets a target opening degree of the valve in response to an operating condition of the engine. It is determined that a malfunction occurs in the EGR when a difference between the actual valve opening degree and the target opening degree is greater than a predetermined value.
The above-mentioned EGR valve is operated by a pressure supplied by a vacuum switching valve (duty-VSV) which is controlled by an EGR computer to switch the pressure between a negative pressure in an intake pipe and an atmospheric pressure with a variable duty ratio. Thus, if atmospheric air passage of the duty-VSV is clogged, the atmospheric air cannot be introduced into the EGR valve. Accordingly, since a negative pressure in the EGR valve cannot be released, the EGR valve is maintained in an open state which results in a close malfunction state.
However, when the EGR valve is not operated for a certain time period, air is gradually introduced through the atmospheric air passage and thus the EGR valve is closed. In a state where the EGR valve is not operated, the target valve opening degree is set to zero. Additionally, when the EGR valve is closed due to introduction of air through the atmospheric air passage as mentioned above, the actual valve opening degree is also zero.
Accordingly, if a determination of malfunctioning is performed in such a state, the difference between the actual valve opening degree and the target valve opening degree is less than a predetermined value. Thus, there is a problem in that an erroneous determination is made that the EGR valve is normal even though the EGR valve has a cause of malfunctioning due to the atmospheric air passage being clogged.